There are many posts about problems between Google Assistant and spotify but they are usually from 2018 so the software was completely different back then. 

The problem however is the same. Suddenly after days of happy "spotify on google assistant" then google replies to "play Music" with the normal "Playing Music on spotify" but nothing plays.

I solved it by the following steps however many of these steps may be unnecessary so I would like comments from people who could get it working easier than this.

1. Remove and re-add Spotify to the phone. This is most likely unnecessary but I did it.

2. Go to settings->apps and Notifications->click on spotify and make sure you have "unlimited data access" and that you can access "Storage, microphone, camera". Do the same thing for the (Google)Home app.  Again this is probably unnecesary but I did it because this was the recommendations from 2018 posts.

3. A restart of your phone is probably smart here at some point.

4. This last step is what seemed to really make it start working. I started the (Google)Home app and in it click on "media" and then "Music" and then remove the link to Spotify. Then I re-added the link to spotify. 

Yes! It started working at this point once again. I suspect also that a major problem could be not having the right to "unlimited data access." I think it just went over the limit of allowed data access and then stopped working but I am not sure.